In 3GPP Long Term Evolution/3GPP Long Term Evolution-Advanced (LTE/LTE-A), with continuous rapid growth in a quantity of antennas at a data transmit end, a quantity of to-be-served user equipments (UEs), that is, a quantity of to-be-scheduled UEs, is also growing rapidly. More antennas can provide a higher degree of spatial freedom. This creates a favorable condition for multiplexing a plurality of data streams (which may be single-user multiple-input and multiple-output (SU-MIMO) or multi-user multiple-input and multiple-output (MU-MIMO)) in downlink space.
To obtain a high degree of spatial freedom that can be provided by large-scale antennas, a data transmit end (usually, a base station) needs to obtain related channel state information (CSI), so that a precise precoding matrix (precoder) is obtained. In the prior art (for example, LTE/LTE-A), during MIMO, a data transmit end usually obtains CSI using two methods.
One method is: In Time Division Duplex (TDD)/Frequency Division Duplex (FDD), a data transmit end sends a pilot for measuring downlink CSI, a data receive end (usually, UE) measures the pilot to obtain the CSI, the UE feeds back the CSI (usually, quantized CSI, which is PMI+RI in LTE), and the data transmit end precodes data using the CSI and sends precoded data. The other method is: In TDD, a data receive end sends a pilot (for example, an SRS in LTE/LTE-A) for measuring uplink CSI, a data transmit end measures uplink channel CSI, the data transmit end determines, according to channel reciprocity, that uplink channel measurement is equivalent to downlink channel measurement (usually, calibration needs to be performed using a necessary reciprocity parameter), and then the data transmit end precodes data using the CSI and sends precoded data.
Because downlink pilot overheads are directly proportional to a quantity of antennas at a data transmit end, uplink pilot overheads are directly proportional to a quantity of to-be-served UEs, and a quantity of uplink CSI feedbacks is also directly proportional to the quantity of antennas at the data transmit end, when there are not many antennas (for example, four/eight antennas in LTE/LTE-A) at the data transmit end, the pilot overheads and the quantity of uplink CSI feedbacks can be controlled. However, when there are many antennas (a quantity of UEs that can be scheduled also increases), the uplink and downlink pilot overheads and the quantity of uplink CSI feedbacks occupy a large quantity of time-frequency resources. As a result, there are fewer time-frequency resources available for data transmission, and a system throughput is greatly affected.